The present invention relates to color circuits of a color television receiver and, more particularly, to an automatic chroma control circuit.
Generally, in color circuits of color television receivers, a carrier chroma signal is separated from a composite color video signal, and color signals applied to a color cathode-ray tube are demodulated. The color circuits comprise, as shown in FIG. 1, a first chroma amplifier 11 (band-pass amplifier) for extracting a carrier chroma signal from a composite color video signal, a second chroma amplifier 12 (band-pass amplifier) connected to first chroma amplifier 11, color demodulators (not shown) connected to second chroma amplifier 12 which demodulate color signals, a burst amplifier 13 for extracting color bursts from the carrier chroma signal in response to gate pulses such as horizontal flyback pulses, a voltage-controlled oscillator (VCO) 14 for generating a color subcarrier signal, and an automatic phase control (APC) circuit 15 for controlling the output frequency and phase of VCO 14 in response to the separated color sync bursts. For obtaining a good color picture, the color circuits further comprise a color killer detector 16 responsive to the burst amplifier 13 to cut off second chroma amplifier 12 when a chroma signal is absent or is very small, and an automatic chroma control (ACC) circuit (ACC detector circuit) 17 responsive to the burst amplifier 13 to reduce the gain of first chroma amplifier 11 as the carrier chroma signal received becomes greater so that the output level of the carrier chroma signal is maintained constant.
In FIG. 1, color killer detector 16 is of synchronous detection type which compares in phase the color sync burst signal and the output signal from VCO to produce a color killer voltage. ACC detector circuit 17 is of peak detection type which detects a peak value of the color sync burst signal. Peak ACC detector circuit 17 is susceptible to influence of noise. Accordingly, when a color signal is small, the color signal is made smaller since ACC detector circuit 17 operates from noise generated in the color television receiver. Therefore, a color missing signal level becomes adversely higher than that under the condition in which there is no noise, as shown in FIG. 2. In FIG. 2, curve a shows input/output characteristics of the color signal processing circuit when there is noise, while curve b shows input/output characteristics when there is no noise.